Crews search for the swimmer in the wake of his disappearance. Photo: Ten News

Police believe the Kenyan-born South Australian man was swimming with a friend at Surfers Paradise beach in waist-deep water about 12.30am on Monday morning when a wave knocked them down. The 19-year-old never resurfaced and his friend searched for about half an hour before calling police.

Police and surf life savers used helicopters, rescue boats and beach buggies to comb a large area of land and water for about a day and a half before he was found.

Several members of the man's family flew from Perth to Brisbane when they found out about the search. It's believed they were still on the plane or in the airport when the body was found.

Surfers Paradise police Sergeant John Goobanko said some family members had been told of his death but police weren't ready to release the man's name.

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"We've formally identified the swimmer and we are satisfied that it was the person that it's believed went missing," he said.

"Of course the family are upset about the death of their family member.

"They're receiving support from family and support units."

It's not known how long the body was lying on the shore at South Stradbroke Island before being found, but police had searched the strip of sand in the morning and found no sign of the man.

The coroner will investigate the death.

"We always tell people to swim between the flags, don't swim at night and particularly don't swim if you've been drinking, not that I know he's been drinking, but the investigation will look into whether the man was drinking," Sgt Goobanko said.

"When a person's gone swimming at Surfers Paradise at midnight you'd have to include in your investigations whether alcohol's been consumed."